This invention relates to the accurate measurement of voltage-ampere reactive power (VAR) and more particularly a method and apparatus for measuring VARs in a real world environment where the waveform of interest cannot be accurately approximated as a sinusoid.
Electrical utility system designs are based on the kilovolt-ampere (KVA) load to be served. The KVA load is regarded as the power expended under a given range of power factors. The power factor is defined as the ratio of current drawn by the load to the total current. The total current is defined in terms of kVARs. KVAR metering, or so-called phase-displaced metering, is an attempt to measure voltage and current in quadrature phase relationship to one another, under the assumption that the waveform is sinusoidal (i.e., has no harmonics). The VAR measurement derives from the unit of measurement of magnetizing volt-amperes and is an abbreviation for the phrase "volt-amperes reactive". It is conventionally defined as the voltage times the magnetizing current in amperes.
A theory of power measurement has been developed based on the use of VAR measurements, not all of which is technically correct. Moreover, traditional metering does not yield sufficiently-accurate VAR measurement to permit accurate system design and load management. Traditional measurement techniques involve measuring the current, measuring the voltage at the same time as the current is measured and then delaying the voltage measurements by a time equal to the phase difference of the fundamental frequency. As a consequence, a non-sinusoidal waveform is not accurately depicted and measured by conventional VAR measurement techniques, since a non-sinusoidal waveform contains harmonics of the fundamental which are not properly delayed. As a further consequence, traditional metering techniques may give wildly inaccurate VAR measurements, leading to inconsistent and misleading design and operation decisions, particularly where the voltage and current waveforms are subject to high distortion.
Traditional VAR calculation is described in the Handbook For Electricity Metering published by the Edison Electric Institute (8th Ed.) Chapter 9, pp. 173-196 (first Ed. 1912, latest Ed. 1981). Therein is described the traditional VAR metering techniques and theory of operation. It is of interest to note that the method of measurement of electrical quantities is dependent upon the rate, the cost and the required accuracy. What is needed is a metering technique which is highly accurate under all loading conditions.